1. Field of the Invention
The present invention relates to a radial-flow turbine with an overhung turbine wheel acted upon by a working fluid flowing in the radially inward direction. The turbine wheel includes at least one ring of turbine blades and is connected to a shaft.
2. Description of the Prior Art
A radial-flow turbine with two rings of turbine blades is described in the German Pat. No. 1 144 738. This known radial-flow turbine operates with a high circumferential efficiency for a given steam flow rate, i.e., for a given temperature gradient. This circumferential efficiency nearly corresponds to a single-stage turbine, however, the circumferential speed of operation cannot be significantly less. The turbine blades are always manufactured separately and are mounted in a suitable manner with each blade base received in and joined with the turbine wheel. For reasons of strength, the blade bases and the junction elements must be adequately large, the result being a correspondingly substantial axial length. Consequently, at high speeds of revolution of the turbine wheel, correspondingly large inertial forces arise. In the past, in radial-flow turbines of this design manufactured by the Applicant, it was necessary that the turbine wheels be supported at both shaft ends--this also being the case for the German Pat. No. 1 144 738. These known radial-flow turbines could be used only in exceptional cases, most of all because the elaborate design resulted in high manufacturing costs. Due to the substantial mass of the turbine-wheel, the thermodynamically required circumferential speeds could be controlled in practice only at very high cost.
Moreover, a two-ring radial-flow turbine is described in the German Offenlegungsschrift No. 1 551 190 wherein the turbine wheel is provided with a central bore for the unilaterally supported shaft. The diameter of the central bore and that of the shaft are designed according to required strength computations. Depending on the design angular speed, the size of the turbine, and other design criteria, an optimal diameter is calculated for each individual turbine. Accordingly, the connection between the shaft and the turbine wheel is not suited for standardized radial-flow turbines. Furthermore, plasticization due to stress-focusing may take place in the area of the central bore.
Also, integral turbine wheels for exhaust driven turbochargers are known, which are connected to the shaft by butt-welding. These are described, for example, in the periodical MTZ, 16th year, No. 11, November 1955, page 323, as comprising a turbine wheel in which the blades are moved up as far as the axial exhaust area. The actual centrifugal forces acting on the turbine blades therefore produce practically no bending moments at the transition between the turbine wheel and the turbine blades. This is in contrast to the previously described radial-flow turbines wherein the turbine blades are mounted to at least approximately radial surfaces of the turbine wheel, and the bending loads generated by the centrifugal forces must be absorbed by making the turbine wheel correspondingly thick and wide.
More recent experimentation has shown furthermore that more than trivial damage, mostly in the area of the rings of blades, is incurred due to impurities in the steam fed to the turbine. This condition was found in particular to apply to those steam supply networks which are shut off at specific times, for instance on weekends. While such a condition can be remedied by resorting to impurity separators, filter or the like, such solution demand high capital investment, and also entail flow losses. Accordingly, the intrinsically high economy of the radial-flow turbine of the above discussed typed is rendered problematical, and it would be difficult to expand the fields of their use beyond those already known.